1. Field of the Invention
The invention relates to a method of manufacturing a ferrite magnet for motors, in which method a piece having the form of a segment of a cylinder is produced by pressing ferrite powder in a mould in the form of a segment of a cylinder, whereafter it is sintered, i.e. heated at a high temperature, and finally magnetized by placing it between two dies of a magnetizable material, which are the armatures of an electromagnet and which each contact said piece along one of the cylindrical surfaces.
The invention also relates to a ferrite magnet in the form of a magnetized segment having magnetic-induction lines which are oriented substantially radially with respect to the cylindrical segment.
2. Description of the Related Art
Such a magnet is used as a pole piece in small-power motors, for example for windscreen wipers, electric windows etc. in cars. At present, it is required that such motors produce very little noise. There is a noise which is caused by a sudden variation of the magnetic field at the ends of the pole pieces. This source of noise is commonly referred to as "cogging torque".
According to JP Kokai No. 4-261352, this effect can be reduced by using a magnet whose end portions have gradually decreasing width dimensions in the direction of the rotary shaft of the motor so as to terminate as a spike. However, such a magnet is expensive to manufacture owing to the complex shape of the mould.